The present invention relates to a boom mechanism cooperable with a lift vehicle or base structure and, more particularly, to a boom mechanism including two pivoting boom members liftable in a dependent relationship by a single lifting mechanism.
A conventional straight boom lift typically includes a single telescopic boom by which the platform can be positioned from low angles (usually below horizontal) to high angles (such as around 75xc2x0 above horizontal). Boom angles near horizontal create a situation of forward instability in which the machine may tend to tip toward the platform due to the overhung load created by the platform load and boom assembly. Counterweight is usually added to the tail of the vehicle turntable to counterbalance the destabilizing moment created by the boom and platform load.
Maximum boom angles, on the other hand, tend to create a situation of backward instability in which the weight of the boom and the counterweight in the tail of the turntable tend to cause the machine to tip in the direction opposite the platform. A counterweight added to the frame of the machine helps to counterbalance the destabilizing moment caused by the boom and tail counterweight. The total weight of the machine is then dependent on the compromise made for the placement of weight required to satisfy both conditions of instability.
A conventional single tower articulated boom lift typically includes two booms connected by an upright. The upright is held in a vertical (plumb) orientation as the lower boom or tower is raised in angle. Maintaining the upright in its vertical orientation is usually achieved by a master and slave hydraulic circuit or a parallelogram linkage with the tower boom. The upper boom is typically pinned to the upright with its own lift cylinder, which can be raised or lowered in angle through its full range of motion regardless of the position of the tower boom. As a consequence, the two booms can be independently positioned to allow the machine to be xe2x80x9carticulatedxe2x80x9d into work positions or positioned up and over obstacles. The total maximum height of the platform is achieved by the contribution of the tower and the upper boom lengths. Each boom is typically shorter than the boom of a comparable height straight boom lift; therefore, the maximum horizontal outreach provided by the upper boom is typically less than the single boom of a comparable height straight boom lift.
A position of maximum forward instability for this type of boom lift is encountered when the tower is raised to its full angle with the upper boom near the horizontal angle. This position creates the maximum horizontal outreach of the platform as well as positioning the boom structure weight in the most detrimental position to the forward stability of the machine. Just as in the straight boom designs, counterweight is added to the tail of the turntable to counterbalance the destabilizing moment of the upper boom and the platform load.
A position of maximum backward instability for an articulated boom lift occurs when the tower is lowered to a near horizontal angle while the upper boom is raised to its maximum angle. In this position, the weight of the boom structure has moved to the most detrimental position to the backward stability of the machine. As in the case of the straight boom lifts, the backward instability is made worse by the presence of the tail counterweight added to reduce forward instability. Consequently, similar to the straight boom design, frame counterweight is added to counterbalance the destabilizing moment caused by the boom and tail counterweight.
The boom mechanism according to the present invention does not fall into the category of a straight boom lift or an articulated boom lift. That is, the construction according to the invention is not a straight boom lift as it incorporates the tower boom, upright and upper boom found on a single tower articulated boom lift. Additionally, the construction according to the invention is not an articulated boom lift as the booms cannot be independently positioned with respect to each other. The arrangement incorporates a linkage that mechanically ties the tower boom and the upper boom to each other allowing one lift cylinder to lift the entire boom structure. Thus, one boom cannot be raised without the other also being raised, creating forward and backward instability characteristics that greatly differ from the conventional straight or articulated boom lifts.
A condition of maximum forward instability with the construction according to the invention is forced to occur when the tower is near horizontal rather than at its full angle as in the conventional articulated boom lift. See FIG. 6xe2x80x94the arrow indicating the direction of instability. This construction reduces the horizontal outreach of the upper boom and therefore the degree of destabilizing moment of the upper boom and the platform load. It also allows the weight of the boom structure to be in the most favorable position to aid in the counterbalancing of the upper boom and platform load destabilizing moment. Both of these factors result in less tail counterweight required to counterbalance the boom and platform.
The condition of maximum backward instability for the conventional articulated boom is eliminated (i.e., when the tower is down while the upper boom is at its maximum angle). With the present construction, a position of maximum backward instability occurs when the upper boom is fully raised and, by default, when the tower is fully raised. See FIG. 7xe2x80x94the arrow indicating the direction of instability. This puts the weight of the boom structure in the best possible position to aid in counterbalancing the destabilizing moment caused by the boom and the already reduced weight tail counterweight. The result is a dramatic reduction in the need for counterweight in the frame.
In accordance with an exemplary embodiment of the invention, a boom mechanism cooperable with a lift vehicle or base structure includes a tower boom pivotally securable at a base end to the lift vehicle or base structure. An upright pivotally secures an upright end of the tower boom. An upper boom is pivotally secured at one end to the upright, and a timing link is connected between the upper boom and the tower boom. A lift cylinder is connected between the upright and the upper boom. The upright may have a fixed orientation relative to the vehicle or base structure. In this context, the boom mechanism may also include a tower link pivotally attached at one end to the lift vehicle or base structure and pivotally attached at an opposite end to the upright, wherein the tower link fixes the orientation of the upright relative to the lift vehicle or base structure. The upright end of the tower boom may be secured to the upright at a tower boom nose pivot, wherein the timing link is secured to the tower boom at a position spaced from the tower boom nose pivot such that the timing link generates a moment about the tower boom nose pivot.
The upper boom is preferably pivotally secured to the upright at an upper boom pivot, wherein an extension axis of the lift cylinder is spaced from the upper boom pivot such that the lift cylinder generates a moment about the upper boom pivot. The timing link is preferably secured to the upper boom at a position spaced from the upper boom pivot such that a linking force is generated in the timing link as the upper boom is pivoted about the upper boom pivot. The space between the timing link and the tower boom nose pivot may be larger than the space between the timing link and the upper boom pivot, thereby creating a mechanical advantage to assist in lifting the tower.
The timing link is preferably secured to the upper boom in a position that effects displacement in one direction relative to an orientation of the timing link at low angles with a component in a substantially perpendicular direction that increases with increasing extension of the lift cylinder. On the other hand, the timing link is preferably secured to the tower boom in a position that effects displacement in the substantially perpendicular direction relative to the orientation of the timing link at low angles. In a preferred arrangement, the lift cylinder is the only motive force of the boom mechanism.
In another exemplary embodiment of the invention, a lift vehicle includes a vehicle chassis supporting a plurality of wheels, a drive system operable for driving the wheels, a base structure supported by the vehicle chassis, and the boom mechanism according to the invention secured to the base structure.
In accordance with yet another exemplary embodiment of the invention, a boom mechanism includes a tower boom and an upper boom liftable in a dependent relationship by a single lifting mechanism. The tower boom and the upper boom are respectively pivotally secured to an upright. A timing link is secured between the tower boom and the upper boom, and the lifting mechanism is secured between the upright and the upper boom.
In accordance with still another exemplary embodiment of the invention, a method of constructing a boom mechanism cooperable with a lift vehicle or base structure includes the steps of providing a tower boom pivotally securable at a base end to the lift vehicle or base structure, pivotally securing an upright end of the tower boom to an upright, pivotally securing an upper boom at one end to the upright, connecting a timing link between the upper boom and the tower boom, and connecting a lift cylinder between the upright and the upper boom.
In accordance with another exemplary embodiment, a boom mechanism includes a tower boom and an upper boom liftable in a dependent relationship by a single lifting mechanism, with the tower boom and the upper boom being respectively pivotally secured to an upright. A timing link is secured between the tower boom and the upper boom, and the tower boom is shorter than the upper boom. The tower boom and the upper boom are preferably the only booms of the boom mechanism. The upper boom may be a telescopic boom.